The invention relates to electrical precipitators, and more particularly to a method and apparatus to insulate and prevent sparking of the high voltage supply electrodes in an electrical precipitator.
Generally, electrical precipitators are used to remove pollutants from the exhaust created by burning fossil fuel. Specifically, in some energy generating power plants, electrical precipitators are used to remove fly ash produced by burning coal as a power source from the exhaust, or flue gases. With increasing regulations on the discharge of by-products such as fly ash into the atmosphere, it is desirable to increase the efficiency of existing precipitators, and produce new precipitators with higher performance levels.
Typically, an electrical precipitator has an array of discharge electrodes which are vertically suspended from a hanger frame in what is known as a precipitator box. The hanger frame is connected to a high voltage supply electrode extending vertically out of the precipitator box and through a precipitator roof which is at electrical ground. The supply electrode is electrically charged by a precipitator transformer and controlled by an automatic voltage controller. Typically, rapper rods are positioned inside the supply electrodes and are activated by a set of rappers, which may comprise an activation solenoid. An array of ground plates are also situated in the precipitator box parallel to the discharge electrodes and have separate rappers and rapper rods. The rapper rod and supply electrode combination extend through a support housing attached to the precipitator roof. Other rapper rods may be provided without supply electrodes and extend through nonsupport housings to the hanger frame.
In operation, the exhaust or flue gases produced by the coal burner are passed at right angles through the field of discharge electrodes and ground plates. As the fly ash in the exhaust moves through the field, the particles become electrically charged and are attracted to the ground plates and discharge electrodes. Periodically, as the ground plates and discharge electrodes become covered with fly ash, the rapper rods are activated by the rappers in the absence of an electric charge and physically jar the hanger frames to dislodge the fly ash and cause the particles to fall to the bottom of the precipitator box where they are later removed.
To increase the efficiency of the electrical precipitators, manufacturers have increased the supply voltage of the electric charge to levels where 70,000-75,000 volts have become common. The increased voltage to the discharge electrodes is designed to charge more of the fly ash particles in the precipitator box and therefore remove more fly ash from the flue gases. However, certain high voltage precipitators have not been performing as well as expected because the supply electrodes are sparking to the grounded precipitator roof and dissipating the higher voltage levels. Sparking prevents the discharge electrodes from reaching the high voltage potential supplied to the supply electrodes. As a result, the automatic voltage controller attempts to raise the voltage higher causing more frequent sparking from the supply electrodes to the precipitator roof and results in even lower voltage to the discharge electrodes in the precipitator box. Although the supply voltage is higher, the end result in overall precipitator performance is increasingly deteriorated. Further, even though some of the rapper rods do not have supply electrodes, they are connected to the hanger frame and do become electrically charged. Therefore, these rapper rods are subject to sparking and add to the above described problem.